1. Field of the Invention
The present invention relates to a continuous process for the grafting of ethylenically unsaturated monomers onto polyolefins and especially to the grafting of ethylenically unsaturated carboxylic acids and anhydrides onto homopolymers of ethylene and copolymers of ethylene and higher alpha-olefins.
2. Description of the Art
Polymers of alpha-olefins in which the alphaolefin is a hydrocarbon are well known. Since polymers, especially homopolymers of ethylene and copolymers of ethylene with C.sub.4 -C.sub.10 alpha-olefins, are used in large volumes for a variety of end-uses e.g. in the form of fibres, films, moulded articles and the like.
Polyolefins are relatively non-polar, which is an important and beneficial characteristic for many end-uses. However, in some instances the non-polar nature of polyolefins may be a disadvantage. For instance, polar additives have a tendency to exude from polyolefins and it is usually necessary to treat polyolefins to promote adhesion to adhesives and printing inks.
It is known that the properties of polyolefins may be modified by the grafting of ethylenically unsaturated carboxylic acids and anhydrides onto hydrocarbon polyolefins. In U.S. Pat. No. 2,973,344, which issued Feb. 28, 1961 E. V. Fasce describes the grafting of unsaturated alpha-beta dicarboxylic acid anhydrides onto low pressure polyalkene polymers, including grafting in the absence of solvent at 121.degree.-204.degree. C. W. C. L. Wu et al. describe the grafting of cyclic ethylenically unsaturated carboxylic acids and anhydrides onto polyethylene, under melt conditions and in the presence of t-butyl hydroperoxide, including the grafting of maleic anhydride on to polyethylene in a comparative example, in U.S. Pat. No. 3,873,643, which issued Mar. 25, 1975 R. A. Steinkamp et al describe, in Canadian Pat. No. 993,592 which issued July 20, 1976, a process for modifying the rheological or chemical and rheological properties of a polymer (polypropylene) by injecting a modifier and/or a free radical initiator into molten polymer in an intense mixing and reaction zone within an extruder; shear degradation of the polymer occurs. A process which comprises mixing polyethylene under melt conditions at 140.degree.-210.degree. C. with an ethylenically unsaturated polycarboxylic acid anhydride modifier is described in Canadian Pat. No. 1,109,183 of R. T. Swiger et al., which issued Sept. 15, 1981. The grafting of fumaric acid diester onto polyethylene, preferably at 270.degree.-320.degree. C., is described in Canadian Pat. No. 867,818 of R. J. Zeitlin, which issued Apr. 6, 1976. Such processes tend to use relatively high levels of organic peroxide in the grafting process.